The following U.S. patents are representative of the most relevant prior art known to the applicant at the time of the filing of this application:
______________________________________ 2,279,260 Benner et al April 7, 1942 2,911,313 Sandmeyer November 3, 1959 3,093,498 Whittemore et al June 11, 1963 3,888,687 Manigault June 10, 1975 3,948,670 Manigault April 6, 1976 4,125,407 Ueno November 14, 1978 4,125,409 Friedrichs November 14, 1978 4,126,654 Montgomey et al November 21, 1978 4,217,113 Suh et al August 12, 1980 4,222,782 Alliegro et al September 16, 1980 4,235,636 Friedrichs et al November 25, 1980 ______________________________________
A refractory liner is needed in gasifier furnaces used for converting coal or petroleum coke or other ash containing sources of carbon into methane for use in the chemical industries and in glass melting furnaces. While somewhat satisfactory liners are used today, the slag produced from melting various impurities in the source of the carbon in such furnaces, have been found to be very difficult to contain. The conventional refractory liners for such furnaces now have as much as 20% porosity in the form of interconnected passages into which the molten slag and glass penetrate to ultimately mechanically and chemically weaken the liner structure causing it to crack, spall, and erode.
It is therefore desireable that an improved refractory liner be made available, e.g. for gasifier furnaces, that is more resistant to the corrosive and erosive action of slag, and which in addition to being able to contain the chemically reacting carbon compositions and the slag generated as the gasification process continues, also serves as an efficient thermal barrier at the furnace wall when the liner is subjected to temperatures in the range of from 500.degree. C. to 1800.degree. C.
It is the purpose of this invention to provide an improved refractory liner material in the form of an alumina-chromia solid solution that can stand up to the high temperature conditions and the chemical and mechanical slag and molten glass containment problems. For this purpose the present disclosure encompasses a unique, uniformly dense, alumina-chromia solid solution refractory product that is better adapted to contain the molten glass and slag generated in the operation of gasifier furnaces. The tile or brick described herein has a composition that is chemically and mechanically resistant to the corrosive and debilitating action of the slag as well as providing a satisfactory thermal barrier for lining the furnace. The method of forming and firing the tile is also disclosed.
Concerning the above mentioned prior art, the patents to Benner et al and Sandmeyer show examples of products making use of a fusion process to make alumina-chromia solid solutions. Benner et al forms such a refractory product by fusing alumina and naturally occurring chromite together. In his final product the impurities present in the chromite ore are distributed throughout the body of the refractory. It is proposed that not more than 20% of the fused mass be formed of spinel and his invention is concerned with the control of the level of the impurities to counter the spinel or glassy phase impurities in interstitial areas.
Sandmeyer shows a similar fused cast alumina-chromia solid solution product which in his disclosure is said to be useful for refractory purposes. He first roasts his chromite ore and then fuses that chromite ore with alumina. This, it is said, leads to a denser product which has fewer shrinkage cavities and yet as shown in FIG. 1 of his patent, a rather large cavity and several smaller cavities appear within the cast body.
The two Manigault, Friedrichs '409, and the Montgomery et al patents all teach the manufacture of refractory products which are made with mixtures of rather coarse alumina particles together with chromia and usually other impurities. Manigault, for example, uses tabular alumina grains that fall in a size range of smaller than one quarter of an inch but larger than 325 mesh. He also discusses the presence of a phosphate compound, zircon, clay, and phosphoric acid in the mix he fires to produce his product.
Likewise Friedrichs '409 discloses a refractory product having an alumina matrix that may include tabular alumina to form a matrix to support a solid solution of alumina-chromia in a refractory ramming cement. The large sized alumina grains falling in the range of 4 to 60 mesh and larger if tabular alumina is used, are predominant in the mix and form a strong skeletal structure. The product also includes a glassy phase to form a seal for closing pores and shrinkage cracks. Also a phosphate is used in conjunction with plastic clay to form a preliminary binder to hold the ramming cement together during the firing process.
Montgomery shows an alumina-chromia mixture formed by combining 16 to 54 Tyler mesh alumina grains and chromium metal powder bonded with calcium aluminate. A molded mass of such a mix is then fired to form chrome oxide in-situ. Also an alumina-chromia mix together with chromium powder can be made and bonded with calcium aluminate and then fired.
Suh, Alliegro, and Friedrichs '636 all show other variations of an alumina-chromia fired product. Suh, for example, describes a ceramic tool made by operating upon alumina particles and a reactive chromium metal component to form a multi-phase alumina-chromia composition. The ultimate structure is described as a macrostructure of aluminum oxide which is homogeneous and a microstructure which is non-homogeneous wherein there are ceramic phases.
Alliegro describes a ramming mix that includes alumina-chromia particles in a size range of 4 to 16 mesh and aluminous powder that is oxidized in-situ. The powder is held in its rammed position with a glassy phase alumina-clay constituent and the resulting fired product is described as having as much open porosity as 17% by volume.
Friedrichs '636 shows the use of an essentially alumina-chromite grog that upon being fired forms an alumina-chromia solid solution having all the chromite impurities dispersed therethrough. This fired mass is then crushed to a size range of -3 to 140 mesh grains and is mixed with more chromite in a size range of under 325 mesh and finely ground alumina having a size under 325 mesh. This mixture of larger mesh fused and crushed chromite and the subsequently added fine mesh chromite and alumina is sintered and the fine mesh chromite and alumina form a solid solution matrix and bond between the previously crushed and larger sized grog particles.
Whittemore et al, so far as that reference is relevant to the present invention, discloses a hot-pressed aluminum oxide product which may include, inter alia, chromium oxide. The reference states that the alumina product may contain up to 2% of Cr.sub.2 O.sub.3 or even more, forming a solid solution with the alumina. There is no teaching of anything close to the 10% minimum of the present invention, and it suggests that the material is useful as a cutting tool not a refractory for retaining slag in such an application as a coal gasifier.
In the examples of cements and cold pressed and sintered tiles or bricks, the products are made with mixtures of coarse and fine grains and the coarse grains retain their identity such that a homogeneous body structure does not exist and there are large grains of at least one phase. In the example of fused cast tiles and bricks, the materials are fused into a homogeneous liquid, but in the cooling cycle of the fused cast product, large crystals are formed in the cooling cycle and because of volume changes, large pores are also present. In fused cast, chromia-alumina will be a solid solution of the two oxides, but the crystal size of the solid solution will be well above 200 microns.
In the prior art examples above, various refractory components are segregated within the mass of the resulting products such that a homogeneous body structure does not exist, or a product with large pores and large crystals is obtained. Large crystals are undesirable in such a product because of the anisotropy of the crystals which create differential thermal expansion within the body making it more susceptible to thermal shock.
Fused cast refractories also are nonhomogeneous in that the grain size and grain shape vary from one part of the structure to another as the result of the cooling process. More particularly it has been found when making sintered products, that whenever grain sizes of an alumina component are substantially larger than 150 U.S. Standard mesh, that usually such grains of alumina sinter to form a matrix or skeletal structure that is a separate and distinct phase provided to support the other components in the final product. It is to be noted that when chromite and other less pure particles are used as the source of the chromia, that the impurities form glassy zones or weak spots between the sintered particles of the resulting product, which impurities are subject to chemical attack by the molten slag and are subject to being washed away by the action of the molten slag of the type found in a gasifier furnace or the like.
As distinguished from such known refractory products, this invention provides refractory bodies that are dense alumina-chromia, zirconia-alumina-silica, zirconia-alumina, or alumina-chromia-zirconia which have grain sizes less than 50 microns and less than 8% total pores throughout their masses.